A radiation curable composition, e.g. a radiation curable inkjet ink, typically comprises a photo-initiator. Upon exposure of said composition, the photo-initiator initiates the curing, i.e. the polymerization, of said composition.
Free radical photo-initiators can be classified as Norrish Type I or Norrish Type II photo-initiators. A Type I photo-initiator undergoes photo-cleavage to yield free radicals while a Type II photo-initiator produces free radicals through an abstraction process, in most cases hydrogen-abstraction. While a Type I photo-initiator produces free radicals through an unimolecular bond cleavage reaction, a Type II photo-initiator forms free radicals in the presence of a second molecule, the so called co-initiator also referred to as polymerization synergist.
A first problem for free-radical polymerization processes, especially with Type I photo-initiating systems, is the inhibition of the polymerization, i.e. decrease of the curing speed, by oxygen. Inhibition by oxygen results not only in a decreased overall curing speed, but can also result in an inhomogeneous curing, e.g. poor surface curing resulting in tacky surfaces.
Amines are known in the art as co-initiators for Type II photo-initiators and to suppress oxygen inhibition of free radical polymerization processes. The role of amines in curing systems has been reviewed, for example by R. S. Davidson in “Radiation Curing in Polymer Science and Technology, Volume III”, edited by J. P. Fouassier et al., Elsevier Applied Science, 1993, pages 153-176.
According to WO 02/46323 ethyl-4-dimethylaminobenzoate or an acryl amine may serve as oxygen scavengers in UV curable inks for printing on Printed Circuit Boards.
U.S. Pat. No. 3,963,771 and U.S. Pat. No. 4,045,416 disclose amine acrylates produced by the reaction of a polyacrylate ester, preferably a diacrylate ester, with an amine having at least one hydrogen attached to the nitrogen atom. Said amine acrylates provide a fast curing speed to radiation curable coating compositions.
In the radiation curable coating compositions of U.S. Pat. No. 5,777,024 and US 2004/0254293 (a) tertiary aliphatic amines, e.g. diethanol amine, (b) aromatic amines, e.g. ethyl-4-dimethylaminobenzoate, (c) (meth)acrylated amines and (d) amino-functional (meth)acrylate resin or oligomer blends may be used as co-initiators.
According to WO 06/041289 Type-II photo-initiators are used in combination with (a) a monomer tertiary amine compound, (b) an oligomer or polymer tertiary amine compound, (c) a polymerizable amino acrylate compound or (d) a polymerized amino acrylate compound or mixtures thereof, in radiation curable compositions suitable for use as coating layers in optical fibre technology.
WO 99/03930 discloses a radiation curable coating composition containing an aliphatic maleimide as photo-active component and a tertiary amine, said amine providing a higher curing speed to the composition. The tertiary amine may be aliphatic (e.g. triethyl-amine) or aromatic (e.g. ethyl-4-dimethylaminobenzoate).
A second problem, associated with co-initiators, e.g. amines, present in a radiation curable composition, may arise when unreacted co-initiator remains in the cured composition. Hydrogen transfer from an amine co-initiator to a Type II photo-initiator is rarely quantitative. The unreacted co-initiator remains mobile in the cured composition and may adversely affect the physical properties of the cured composition or may diffuse out of the cured composition. If the radiation curable composition is printed upon food packaging, extraction of the unreacted co-initiator, also referred to as “extractables”, into the food may cause health risks.
In WO 03/091288 a new class of amine co-initiators, in combination with Type I and/or Type II photo-initiators, is disclosed, said amine co-initiators being trialkylamines having a total of 10 to about 36 carbon atoms, wherein at least one alkyl group has a chain length of at least 8 carbon atoms. According to WO 03/091288 said amine co-initiators, when used in combination with a Type II photo-initiator, provide less “extractables” after curing compared to conventional amines as e.g. N-methyl-N,N-diethanolamine.
WO 99/07746 discloses a radiation curable composition containing a radiation curable resin, a photo-initiator and an amine as co-initiator, characterized in that said amine is a compound containing at least one tertiary amino group and wherein at least one substituent of the tertiary amino group is an aliphatic chain containing at least one electron-withdrawing group. Said amine compound is preferably a branched, highly branched or star-shaped dendrimer comprising at least one tertiary amino group. Said co-initiators, in combination with Type II photo-initiators, provide a higher curing speed and less “extractables”.
EP-A 1 616 922 discloses a radiation curable composition containing a radiation curable resin, a photo-initiator and a co-initiator characterized in that said co-initiator comprises a dendritic polymer core with at least one co-initiating functional group, e.g. aliphatic or aromatic amines, as an end group on the polymer core. The radiation curable compositions according to EP-A 1 616 922 are characterized by a high curing speed and a low amount of “extractables”.
EP-A 1 147 094 discloses multi-functional reactive amine acrylates having a low viscosity, prepared by the reaction of multifunctional (meth)acrylates with cyclic secondary amines. These amine acrylates provide a good curing speed, a higher thermal stability and less “extractables” to radiation curable compositions.